1. Field of the Invention:
The present invention is concerned with a structure of a filter utilizing the propagation phenomenon of surface acoustic waves, for use in general wireless and cabled communications equipment, and broadcasting transmitters and receivers.
2. Description of the Prior Art:
Surface acoustic wave devices utilizing the propagation phenomenon of surface acoustic waves have been developed, and some of them have been placed in practical use to replace the conventional resonators and filters which consist of inductance elements and capacitance elements. For instance, a typical example of the surface acoustic wave filters designed to broaden the band and to reduce the insertion losses is a two-port resonant filter of a so-called ring structure in which a transmitting transducer and a receiving transducer are arrayed in cascade on a surface of a piezo-electric substrate, and two sets of slanted grating pairs are arrayed on both sides thereof. With this structure, however, it is difficult to obtain a specific band width greater than 2%. Another known example of the surface acoustic wave filter is of a construction in which a transmitting transducer and a receiving transducer are arrayed in cascade on the surface of a piezo-electric substrate, and multi-strip couplers and reflecting transducers are arrayed in parallel on both sides.
The present invention is directed to the latter of the abovementioned two examples. With the surface wave filter latter structure of the, however, although the multi-strip coupler functions as a directional coupler, the number of the strip conductors constituting the multi-strip coupler must be very increased to heighten the coupling degree. Therefore, it is difficult to make the filters in small sizes thus offsetting the advantages gained by the use of smaller sizes. Further, with the conventional devices, a sufficient reduction of the insertion losses or broadening of the band has not yet been realized also, the frequency characteristics are not flat, and ripple components develop.